Reversing mechanism centering device for roll grinders



Oct. 20, 1970 E. M. wooDFoRD 3,534,505

REVERSING MECHANISM CENTERING DEVICE FOR ROLL GRINDERS Filed June 13, 1968 6 Sheets-Sheet l BY g M f WM' M TrORNEYS Oct. 20, 1970 E. M. wooDFoRD REVERSING MECHANISM CENTERING DEVICE FOR ROLL GRINDERS Filed June 13. 1968 6 Sheets-Sheet 2 MHH {RMEST M. umoDFoRD BY il 23 ATTORNEYS Oct. 20, 1970 E. M. wooDFoRD 3,534,505

REVERSING MECHANISM CENTERING DEVICE FOR ROLL GRINDERS 6 Sheeizss-Sheecl 5 Filed June 13, 1968 INVENTOR E MEST M. UJOODFORD um, 0256, News @MW ATTORNEYS OCt- 20, 1970 E. M. wooDl-'ORD `3,534,505

REVERSING MECHANISM CENTERING DEVICE FOR ROLL GRINDERS ERNEST M. wooDFoRn ABLWWHMWMS @www ATTORNEYS Oct. 20, 1970 E. M. wooDFoRD REVERSING MECHANISM CENTERING DEVICE FOR ROLL GRINDERS 6 Sheets-Sheet 5 Filed June 13, 1968 ERNESTM; wooDPoRD ATTORNEYS Oct. 20, 1.970 E. M. wooDFoRD 3,534,505

REVERSING MECHANISM CENTERING DEVICE FOR ROLL GRINDERS Filed June 13, 1968 6 Sheets-Sheet 6 O 1 H m |l| 1' m \I a A 92 1 2 a I gli i j N C fr! i 'O \l L I l)- t8 m :o c mr@ l* V 5 D ll l o q' P u:

INVENTOR ERNEST M. UJOODFORD ATTORNEYS 3,534,505 REVERSING MECHANISM CENTERIN G DEVICE FOR ROLL GRINDERS Ernest M. Woodford, Waynesboro, Pa., assignor to Litton Industries, Inc., a corporation of Delaware Filed June 13, 1968, Ser. No. 736,648 Int. Cl. B24b 5 04 U.S. Cl. 51-49 11 Claims ABSTRACT OF THE DISCLOSURE This disclosure relates to a reversing mechanism for roll grinders, which reversing mechanism consists of a reversing member synchronized with the movement of the carriage, but at a reduced scale. The reversing mechanism is actuated by longitudinally spaced dogs to reverse the traverse motor and carriage in accordance with the position of the dogs. Each of the dogs is attached to a manually actuated adjusting screw for determining the zone of movement of the carriage as indicated by a reversing member. A gear having teeth meshed with each of the dog adjusting screws has a pair of proximity switches or other suitable signalling device mounted thereon with the switches being arranged to be actuated by the reversing member when the reversing member is midway between the dogs and the grinding wheel is midway between the ends of the roll being ground to stop the traverse motor. When either adjusting screw is rotated, the dog actuated by that screw will move along the screw. At the same time, the rotation of the screw will result in the rotation of the gear along the other of the adjusting screws, carrying the proximity switches with it. The proximity switches will move endwise in the same direction as an adjusted dog, but only half as far with the result that the switches are always at the midpoint between the two dogs.

This invention relates to reversing mechanism for grinding machines, particularly roll grinders, and more particularly to the traverse reversing mechanism for a roll grinding machine.

In this reversing mechanism, a traversing member, representing the carriage, moves in synchronism with the carriage on a Screw which is rotated when the carriage is traversed. A pair of parallel shafts, each of which carries a positioning yoke and a reversing dog, is manually adjustable to place the dogs in positions to correspond to the ends of the roll being ground and thus limit the movement of the grinding wheel carriage.

In machines for grinding crown or concave rolls, the grinding set-up operation must begin with the grinding wheel at the midpoint of the roll. Ordinarily, determination of the center of the roll requires a certain amount of measuring by the operator.

It is, therefore, a primary object of this invention to provide a reversing mechanism for a roll grinding machine wherein the mechanism has means for automatically positioning the carriage at the midpoint between the points of several of the carriage.

A further object of this invention is to provide means operable when the reversal points of the carriage are changed to make a corresponding change in the position of the means for positioning the carriage at the midpoint of the roll being ground.

Another object of this invention is to provide an alter- United States Patent O "l 3,534,505 Patented Oct. 20, 1970 nate circuit for the traverse motor which includes control means for stopping the motor when the carriage is midway between reversal points.

Basically, thus invention consists of a pair of proximity type limit switches or other suitable signalling device mounted on a centering unit having a helical gear, which is engaged with both of the dog adjusting screws. The dog-actuated means, which represents a carriage, carries the switch actuating elements, the lirst of which reduces the traverse speed and the second of which stops the carriage from traversing.

The centering unit, which includes the proximity switches, is so arranged that it is always in the center or half way between the dogs, irrespective of the shifted positions of the dogs. This is accomplished by having the centering unit move whenever a dog is shifted to a new position, however, the centering unit moves only one half the distance that the dog is adjusted, but in the same direction.

With the above and other objects in view that will hereinafter appear, the nature of the invention will be more clearly understood by reference to the following detailed description, the appended claims and the several views illustrated in the accompanying drawings.

In the drawings:

FIG. l is a plan view of a roll grinding machine in accordance with this invention and shows the general details thereof.

FIG. 2 is a wiring schematic of the general controls for the operation of the reversing mechanism and centering device.

FIG. 3 is an end elevational view of the traverse drive and reversing mechanism with parts of the housing being broken away and shown in section so as to illustrate the details of the drive mechanism therein.

FIG. 4 is a rear elevational view of the mechanism of' FIG. 3 with a portion thereof broken away and shown in section so as to illustrate the details thereof.

FIG. 5 is a transverse sectional view taken along the line 5-5 of FIG. 4 and shows specically the details of the mounting of the dogs and the centering unit and the means for effecting the movement and positioning thereof.

FIG. 6 is a fragmentary transverse sectional view taken along the line 6 6 of FIG. 5 and shows further the manner in which the dogs and centering unit are mounted and driven.

FIG. 7 is a fragmentary longitudinal sectional view taken along the line 7-7 of FIG. 5 and shows further the specific details of the mounting of the dogs, the centering unit and the proximity switches.

FIG. 8 is a enlarged fragmentary sectional view taken along the line 8-8 of FIG. 7 and shows specifically the details of the proximity switches and the relationship of the sensing elements thereto.

FIG. 9 is a fragmentary transverse sectional View taken along the line 9 9 of FIG. 7 and shows the specific details of the drive mechanism for one of the dog positromng screws.

FIG. lO is an enlarged fragmentary transverse sectional view taken along the line 10-10 of FIG. 9 and shows further the details of the drive mechanism for the various screws.

Referring now to the drawings in detail, it will be seen that there is illustrated in FIG. l the roll grinding machine which is the subject of this invention, the roll grinding machine being generally referred to by the numeral 15. The roll grinding machine includes a bed 16 on which a carriage 17 is slidably mounted for longitudinal movement. A grinding wheel 18 is mounted on a spindle for rotation with the spindle being supported by a wheel support 20. The wheel support 20 is tiltably mounted on a sub-base 21 with the sub-base being slidably mounted on the carriage 17 for movement toward and from a roll or workpiece W which is to Ibe ground. The workpiece W is rotatably supported between a headstock 22 and a tailstock 23 with the workpiece W being driven through the headstock by means of a headstoek motor 24.

The grinding machine 15 also includes an operator platform 25 along which there is mounted centralized machine controls for carriage traverse, wheel feed and other machine functions. The machine controls includes an apparatus, generally referred to by the numeral 26, for controlling carriage reversal: The apparatus 26 is geared down to cover the maximum length of traverse of the grinding machine within a much shorter space.

Referring now to FIG. 3, it will be seen that the means for traversing the carriage 17 includes a reversible motor 27 which is suitably coupled to a shaft 28 for driving the same. The shaft 28 carries a driving worm 29 which is meshed with a wormwheel 30. The wormwheel 30 drives the carriage 17 by means of a worm and rack (not shown) similar to that shown in U.S. Pat. No. 1,681,794, granted Aug. 24, 1928, and U.S. Pat. No. 1,904,827, granted Apr. 18, 1933.

At this time it is pointed out that the shaft 28 may also be rotated by turning a handwheel 31 with the handwheel 31 being coupled to the shaft 28 by means of a suitable drive and clutch arrangement, generally referred to by the numeral 32 and not described in detail here.

The apparatus 2-6 for reversing the movement of the carriage 17 includes a pinion 33 which is suitably supported by the carriage 17 and which is in driven meshed engagement with a fixed rack 34 on the rear of the bed 16. The pinion 33 is mounted on the lower end of a vertical shaft 35 which is mounted for rotation within a suitable support 36 carried by the carriage 17. A bevel gear 37 is secured to the upper end of the shaft 35 for rotation therewith. The bevel gear 37 is meshed with a bevel pinion 38 which is supported by a horizontal drive shaft 394 which, in turn, is mounted for rotation by means of a support 40 which is suitably supported by the carriage 17.

The opposite end of the drive shaft 39 is provided with a timing pulley 41 which is coupled by means of a timing belt 42 to a timing pulley 43. The timing pulley 43 is mounted on one end of a screw 44 which, when driven from the drive shaft 39, drives a reversing member 45 (FIG. 5) in timed relation with the movement of the carriage 17.

The direction of the movement of the carriage 17 is effected by changing the direction of rotation of the motor 27 through the actuation of a reversing switch 1LS, which is best shown in FIG. 4. The reversing switch 1LS is actuated Iby a switch cam 46 which is mounted on a rod 47, which rod in turn is mounted for axial movement only, by means of a pair of supports 48. The rod 47 is connected by means of suitable linkage 49 to a lever arm 50 which is keyed to a shaft 51 on which the reversing member 45 is mounted. It is to be noted that the reversing member 45 is slidably mounted on the shaft 51 and moves longitudinally thereon between a pair of reversing dogs 52 and 53 when the carriage 17 is traversing.

Referring now to FIG. 5, it will be seen that the reversing dogs 52 and 53 are mounted for sliding movement and are keyed for rotation with shafts 54 and 55, respectively. The shafts 54 and 55 are driven through a suitable gear train at the end of the apparatus 26 from the shaft 51 in a manner to be described in detail hereinafter. The rotation of the dogs 52 and 53 is timed with the rotation of the screw 44 so that pawls 56 and 57 on the dogs 52 and 53, respectively, are in rotary position to engage abutments 58 and 60, respectively, on the reversing member 45 when the reversing member 45 is opposite one or the other of the dogs 52, 53. Carriage reversal is effected when one of the pawls 56, 57 engages a respective one of the abutments 58, 60. At this time the reversing member 45 will be rocked and, in turn, the shaft 51 will rock so as to turn the arm 50 and shift the cam 46. As described above, the movement of the` cam 46 results in the changing of the position of the reversing switch 1LS.

At this time it is pointed out that the carriage 17 may be reversed manually by shifting a lever `61 which is attached to the shaft 51 adjacent one end thereof, as is best shown in FIG. 6.

It is to be understood that the shafts 54 and 55 are also driven by the pulley 43. With reference to FIGS. 4 and 10, it will be seen that the screw 44 has mounted on the end thereof adjacent the pulley 43 a pinion 62 which is meshed with gears 63 and 64. The gears 63 and I64 are, respectively, secured to hubs 65 and 66 by means of suitable fasteners 67.

The hubs 65 and 66, respectively, support shafts 68 and 69 on which there are mounted for rotation pinions 70 and 71. It is to be noted that the shafts 68 and 69 are partially supported by the respective gears 63 and 64. The pinions 70 and 71 are meshed, with gears 72 and 73 which are keyed on the shafts 54 and 55, respectively. At this time, it is pointed out that the gear 63 and the associated hub 65 are mounted for rotation on the gear 72 by means of suitable bearings 74. In a like manner, the gear 64 and its associated hub 66 are mounted for rotation on the gear 73 by means of like bearings 75.

The pinions 70 and 71 are also meshed with inner teeth 76 and 77 of ring gears 78 and 80, respectively. The gears 78 and 80 also have outer teeth 81 and 82, respectively, the purpose of which will be set forth hereinafter.

At this time it is pointed out that the apparatus 26 includes a suitable housing 83 in which the various shafts of the apparatus are suitably journaled in a conventional manner. In addition, the shafts are fixed against axial movement by suitable securing means which are also conventional. Accordingly, the details of the mounting and positioning of the various shafts will not be described in detail here.

Referring once again to FIGS. 5 and 6, it will be seen that the dogs 52 and 53 are carried by yokes 84 and 85, respectively, which are slidably mounted on the shafts 54 and S5 and have portions thereof in threaded engagement with screws 86 and 87, respectively. At this time it is also pointed out that the reversing member 45 is carried by a yoke 88 which is slidably mounted on the shaft 51 and which is in threaded engagement with the screw 44to effect the traversing movement of the reversing member 45 while permitting the rocking thereof so as to rock the shaft 51 while the yoke 88 remains rotationally statlonary.

Referring now to FIGS. 4, 5 and 9, in particular, it will be seen that the screws 86 and 87 are provided at the right ends thereof with pinions 89 and 90, respectively. The pinions 89 and 90 are meshed with the outer teeth 81 and 82 of the gears 78 and 80, respectively. However, the screws 86 and 87 are normally fixed against rotation and thus, the gears 78 and 80 normally remain stationary while the drive train of the apparatus 26 is being driven through the pulley 43.

It is to be understood that the carriage reversal points are determined by the positions of the dogs 52 and 53 as determined by the positions of the yokes 84 and 85, respectively. The yokes 84 and 85 may be re-positioned either when the carriage 17 is stationay or when the carriage is being traversed. The apparatus 26 includes a pair of handwheels 91 and 92 (FIGS. 4 and 7) which are suitably mounted for rotating shafts 93 and 94 so as to selectively rotate the screws 86 and 87 and reposition the yokes 84 and 8S, respectively. As is best shown in FIG. 5, the shafts 93 and 94 are provided at the lower ends thereof with helical gears 95 and 96 which are meshed with helical gears 97 and 98 which are mounted on the screws 86, 87, respectively. The specific details of the mounting of the handwheels, shafts, etc. is found in FIG. 6 with reference to the handwheel 91 and the Shaft 93. Inasmuch as the specic mounting of the shaft 93 and the driving coupling between the handwheel 91 and the shaft 93 is not a part of this invention, further description thereof is believed to be unnecessary.

It will be readily apparent that when the feed screws 86 and 87 are rotated, they will result in the rotation of the gears 78 and 80 and the shafts 54 and 55 will also be rotated` It will be readily apparent from the foregoing description of the apparatus 26 that during the traversing of the carriage 17, there will be a like proportional movement of the reversing member 45. When the reversing member 45 moves into position for engagement by one of the pawls 56 or 57, the reversing member 45 will be rocked a limited degree due to the engagement of one of the abutments 58, 60 thereof by a respective one of the pawls 56, 57 with the result that the shaft 51 is rocked so as to reverse the position of the limit switch 1L'S. This will in turn reverse the motor 27 so as to reverse the traverse of the carriage 17, as is required in the particular grinding operation.

CENTERING MECHANISM It is frequently necessary to start a grinding operation with the grinding wheel at the longitudinal center of the roll to be ground. This position will vary from roll to roll and is usually determined by direct measurement. The carriage is then traversed by push button controls to the position so indicated by the measurement. This, obviously, is time consuming.

In accordance with this invention, automatic means have been provided for centering the grinding wheel 18 with respect to the particular workpiece W to be ground.

Automatic means for centering the workpiece W relative to the grinding wheel includes a centering unit, which is generally referred to by the numeral 100. The details of the support unit are best seen in FIG. 7 and includes a plate 101 to which there is releaseably secured a pair of proximity limit switches 102 and 103. The plate 101 is secured by means yof screws 104 to a flanged portion of a hollow shaft 105. A generally cup shaped helical gear 106 is rotatably mounted on the intermediate portion of the shaft 105 with the helical gear 106 being positioned to be constantly meshed with the screws 86 and 87 of the aforedescribed reversing mechanism. The shaft 105 of the centering unit 100 is xedly secured to a carrier 107 by means of a lock nut and lock washer assembly 108 threaded on the lower end of the shaft 105. An intermediate web portion of the housing 83 has secured thereto a pair of combination guides and supports 109 which cooperate with the housing 83 to define a track for the carrier 107.

It will be readily apparent that when the screw 86, for example, is rotated to adjust the position of the dog 52, it causes rotation of the gear 106 with the result that the gear 106 rolls on the screw 87 and moves the carrier 107 longitudinally of its respective track. It is also to be understood that the carrier 107 and the centering unit 100 carried thereby will move in the same direction as the dog 52, but only half as far. In a like manner, when it is desired to adjust the position of the dog 53, the screw 87 is rotated with the result that the gear 106 is rotated and rolls along the screw 86 to move the centering unit 100. In this manner, the proximity switches 102, 103 are always midway between the dogs 52, 53.

The activating elements for the proximity limit switches 102 and 103 includes a pair of blades 110, 111, respectively, which are suitably secured to the yoke 88. With 6 reference to FIG. 8, it will be seen that the blades 110 and 111 are of different congurations so as to produce different results through their association with the proximity limit switches 102, 103, respectively. The blade 110 is much wider than the blade 111. As a result, when the machine is set for centering, as the center of the roll to be ground approaches alignment with the grinding wheel 18, the reversing member 45 and the yoke 88 will also approach its centering position. At this time, the wide blade 110 will provide a magnetic impulse of the proximity switch 102 to provide for the slow down of the movement of the carriage 17 in a manner to be described hereinafter. Thereafter, when the narrow blade 111 reaches its centered position with respect to the proximity limit switch 103, the necessary magnetic impulse will be provided to operate the switch and stop the carriage 17 at the midposition of the roll W.

OPERATION With reference to FIG. 2, it will be seen that normal operation of the grinder 15 is initiated by pushing the traverse start push button switch PB-13 to complete a circuit through normally closed traverse stop push button switch PB-12 to energize relay SCR. At this time, holding contact 3CR1 closes to complete a holding circuit through normally closed contact 42CR1.

The energization of relay 3CR also results in the cl-osing of Contact SCR2 to complete a circuit through then closed contact 1LS1 to energize relay 6CR. At this time contact 6CR2 closes to complete a circuit through normally closed contact SCR2 to energize relay 7CR which controls the operation of the motor 27 so as to drive the carriage 17 towards the headstock. The energization of relay 7CR also results in the closing of contact 7CR2 so as to complete a holding circuit through normally closed Contact SCRL When the relays 6CR and 7CR are energized, normally closed contacts 6CR1 and 7CR1 in the circuit to the relay SCR open assuring the de-energizing of relay SCR, which relay controls the operation of the motor 27 in a direction to move the carriage 17 towards the footstock.

As has been set forth above, the motor 27 is connected through the shaft 28 to effect carriage traverse movement. As the carriage 17 moves towards the headstock, the rack 34 on the rear of the bed l16 rotates the pinion 33 which, through the associated gear train, results in the rotation of the timing pulley 43 and the screw 44. At the same time, as is previously described, the shafts 54 and 55 rotate to turn the associated dogs 52 and 53'.

As the carriage 17 is traversed towards the headstock, the reversing member 45 is traversed longitudinally to the right in FIG. 5 due to the movement of the yoke 88 on the screw 44. This longitudinal movement of the reversing member 45 continues until the pawl 57 on the rotating dog 53 engages the abutment 60 to rock the reversing member 45. This, in turn, results in the rocking of the shaft 51 to which the reversing member 45 is keyed. When the shaft 51 rocks, the arm 50 is pivoted and through the associated linkage attached thereto, the cam 46 is shifted so as to reverse the position of the reversing switch ILS with the result that the positions of the contacts 1LS1 and 1LS2 are reversed, the contact 1LS1 opening and the contact 1LS2 closing.

When the position of the reversing switch lLS is reversed, the circuit to the relay GCR is opened and the normally closed contact L6CR1 again closes. At the same time, the circuit to the relay SCR is completed with the result that contact SCR2 is closed so as to complete a circuit through normally closed contact 7CR1, which has again closed, to energize relay SCR. Holding contact 5CR1 now closes to complete the holding circuit through normally closed contact `SCRL Relay lSCR controls the rotation of the motor 27 in the reverse direction with the result that the carriage 17 now traverses towards the tailstock.

When it is desired to center the roll to be ground relative to the grinding wheel prior to the initiation of the grinding operation, a three position selector switch SS1 is moved to the centering position thereof. In this position of the selector switch SSI, a circuit is completed to energize relay 39CR. At this time, contacts 39CR1 and 39CR2 in the circuits for relays 41CR and 42CR, respectively, close. At this time, it is pointed out that the relay 41CR is suitably coupled in the control circuit for the motor 27 so as to slow down the rotation of the motor. In a similar manner, relay 42CR is coupled in the electrical circuit for the motor 27 so as to stop the rotation of the motor 27.

Assuming the carriage 17 to be already moving towards the center position for the particular roll to be ground, it will be apparent that the reversing member 45 is moving longitudinally towards the centering unit 100. As the carriage 17 approaches the center' position of the roll, the slow down blade 110 actuates the proximity switch 102 to close slow down contact 120, thus completing the circuit to relay 41CR and reducing the speed of the traverse motor 27 and the carriage 17. The carriage 17, however, continues to move towards the center position of the roll, but at a slower rate, until the stop blade 111 actuates the proximity switch 103 to close stop contact 121, completing the circuit to energize relay 42CR. At this time normally closed relay 42CR1 opens, deenergizing relay 3CR to stop the traverse motor 27 and the carriage 17. The grinding wheel 18 will now be located at the center of the workpiece or roll W.

At this time it is pointed out that the wiring schematic also shows associated with the selector switch SS1 a relay 40CR and contacts 40CR1 and 40CR2. These have to do with other functions of the grinder 15 and have been illustrated only for the purpose of showing the possi- Y bility of three positions of the selector switch SSl. Accordingly, no further description of the relay 40CR and its associated contacts will be made here.

It will be readily apparent that the operation of the centering mechanism of the grinder y15 is automatic. All that is required is that the operator properly position the dogs 52 and 53 in accordance with the ends of the roll or other workpiece W to be ground. Inasmuch as the positioning of the dogs 5-2 and 53 is necessary in order to provide for the proper reversal of the traverse of the carriage 17, no additional work is required on the part of the machine operator to set the centering unit 100. As a result, the only additional function required of the machine operator is the positioning of the selector switch SSI prior to the initiation of the grinding operation to make certain that the center of the roll or workpiece W being ground is aligned with the grinding wheel 18. The operation of the centering device is otherwise automatic.

Although only a preferred and specific example of the centering mechanism has been illustrated and described herein, it is to be understood that minor variations may be made in the structure and mounting thereof, without departing from the general spirit of this invention.

I claim:

1. ln a machine tool,

(a) a base,

(b) a tool support slidably mounted on said base,

(c) a tool mounted on said tool support,

(d) a work support having means for rotatably supporting a workpiece,

(e) means for effecting relative transverse and longitudinal movement between said supports,

(f) means for reversing the direction of said longitudinal movement including (g) apparatus driven by said means for effecting longitudinal movement and including (h) a movable first member automatically movable to a position corresponding to the relative position of said tool,

(i) normally fixed second members corresponding to the limits of said longitudinal movement, and

(j) means for individually adjusting the positions of said second members,

(k) centering means for automatically stopping said longitudinal movement when said first member is centered relative to said second members, and

(l) means operable for automatically shifting said centering means in response to adjustment of positions of said second members to maintain said centering means in a centered position relative to said second members.

2. A machine tool according to claim 1 wherein said centering means includes a signalling device operable when the position of said rst member coincides with said centering means to stop said longitudinal movement.

3. A machine tool according to claim 1 wherein said centering means includes a first signalling device operable when said first member approaches a position coinciding with said centering means to slow down said longitudinal movement and a second signalling device operable when the position of said rst member coincides with said centering means to stop said longitudinal movement.

4. A machine tool according to claim 1 in which said centering means has a signalling device operable when said tool and said work support approach a centered relation relative to one another to first slow down and then stop said longitudinal movement.

5. A machine tool according to claim 1 in which the tool support is mounted on a carriage for longitudinal movement on said base.

6. A machine tool according to claim 1 wherein (a) said means for individually adjusting said second members includes a rotatable adjusting screw for each second member,

(b) means mounting said centering means for sliding movement along a path disposed adjacent the path of movement of said rst member, and

(c) said centering means including means in operative engagement with both of said adjusting screws whereby adjustment of one of said second members through a distance will automatically result in the shifting of said center means half said distance in the same direction.

7. A machine tool according to claim 6 wherein said adjusting screws are operable independent of the means for moving said rst member and during said longitudinal movement.

8. A machine tool according to claim 1 wherein (a) said centering means includes a pair of proximity switches operable to control said longitudinal movement,` and (b) said first member carries blades for effecting the operation of said proximity switches.

9. A machine tool according to claim 1 wherein (a) said centering means includes a pair of proximity switches operable to control said longitudinal movement, and

(b) said first member carries blades for effecting the operation of said proximity switches,

(c) said proximity switches being identical and positioned in alignment transversely of the direction of movement of said first member with one of said proximity switches being operable to slow down said longitudinal movement and the other of said proximity switches being operable to stop said longitudinal movement, and

(d) said blade associated with said one proximity switch being wide as compared to the other of said blades whereby slowing down of said longitudinal movement occurs shortly before said other blade is centered relative to said other proximity switch.

10. A machine tool according to claim 1 wherein said machine tool is a grinding machine and said tool is a grinding wheel rotatably mounted on said tool support.

11. A machine tool according to claim 1 together with (a) a control circuit operative for controlling the operation of said means for eifecting relative longitudinal movement of said supports,

(b) said control circuit including a selector switch operable to control the operation of said means for effecting relative longitudinal movement of said supports during both a normal machining operation and a centering operation.

References Cited UNITED STATES PATENTS 11/1921 Conners 51-95 8/ 1928 Stoner 51--49 4/1933 GOOd 51-49 5/1963 HOld et al. 51-49 X U.S. C1. X.R. 

